1. Field of the Invention
The present invention relates to a projection system in which imaging light emitted from a projector is projected on a projection screen to produce thereon an image. More particularly, the present invention relates to a projection screen capable of sharply displaying an image and of providing high image visibility, and to a projection system comprising such a projection screen.
2. Description of Related Art
A conventional projection system is usually as follows: imaging light emitted from a projector is projected on a projection screen, and viewers observe the light reflected from the projection screen as an image.
Typical examples of materials for projection screens for use in such conventional projection systems include white-colored paper or cloth materials, and plastic films coated with white inks that diffuse light. Besides, high-quality projection screens that comprise diffusing layers containing beads, pearlescent pigments, or the like, capable of controlling the diffusion of imaging light, are now commercially available.
Since projectors have become smaller in size and moderate in price in recent years, demand for household projectors such as projectors for family theaters is growing, and an increasing number of families are now enjoying projection systems. Household projection systems are often placed in living rooms or the like, which are usually so designed that environmental light such as sunlight and light from lighting fixtures come in abundantly. Projection screens for use in household projection systems are, therefore, required to show good image display performance even under bright environmental light.
It is however not easy for the above-described conventional projection screens to show good image display performance under bright environmental light because they reflect not only imaging light but also environmental light such as sunlight and light from lighting fixtures.
In such a conventional projection system, differences in the intensity of light (imaging light) projected on a projection screen from a projector cause light and shade to form an image. For example, in the case where a white image on a black background is projected, the projected-light-striking part of the projection screen becomes white and the other part becomes black; thus, differences in brightness between white and black cause light and shade to form the desired image. In this case, in order to attain excellent image display, it is necessary to make the contrast between the white- and black-indication parts greater by making the white-indication part lighter and the black-indication part darker.
However, since the above-described conventional projection screen reflects both imaging light and environmental light such as sunlight and light from lighting fixtures without distinction, both the white- and black-indication parts get lighter, and differences in brightness between white and black become small. For this reason, the conventional projection screen cannot satisfactorily provide good image display unless the influence of environmental light such as sunlight and light from lighting fixtures on the projection screen is suppressed by using a means for shading a room, or by placing the projection screen in a dark environment.
Under these circumstances, studies have been made on projection screens capable of showing good image display performance even under bright environmental light. There have so far been proposed projection screens using, for example, holograms or polarized-light-separating layers (see Japanese Laid-Open Patent Publications No. 107660/1993. (Patent Document 1) and No. 540445/2002 (Patent Document 2)).
Of these conventional projection screens, those ones using holograms have the advantage that the white-indication part can be made lighter if their light-diffusing effect is properly controlled, so that they can show relatively good image display performance even under bright environment light. However, holograms have wavelength selectivity but no polarization selectivity, so that the projection screens using holograms can display images only with limited sharpness. Moreover, because of production problems, it is difficult to produce large-sized projection screens by making use of holograms.
On the other hand, on the above-described conventional projection screens using polarized-light-separating layers, it is possible to make the white-indication part lighter and the black-indication part darker. Therefore, these projection screens can sharply display images even under bright environmental light as compared with the projection screens using holograms.
Specifically, Patent Document 1 describes a projection screen for which a cholesteric liquid crystal that reflects red, green and blue light (right- or left-handed circularly polarized light) contained in imaging light is used in order to make, by making use of the circularly-polarized-light-separating property of the cholesteric liquid crystal, the projection screen not reflect nearly half of the environmental light incident on the screen.
However, in the projection screen described in Patent Document 1, since the cholesteric liquid crystal is in the sate of planar orientation and causes specular reflection when reflects light, it is difficult to recognize the reflected light as an image. Namely, in order to recognize the reflected light as an image, the reflected light is needed to have been diffused, but this point is not taken into account in the projection screen described in Patent Document 1.
On the other hand, Patent Document 2 describes a projection screen using, as a reflective polarization element, a multi-layered reflective polarizer or the like having diffusing properties. This projection screen does not reflect part of the environmental light incident on it because of the polarized-light-separating property of the multi-layered reflective polarizer or the like, and diffuses the reflected light by interfacial reflection that occurs at an interface between materials having different refractive indices, constituting the multi-layered reflective polarizer, or by means of a diffusing element that is provided separately from the multi-layered reflective polarizer. Patent Document 2 also describes a projection screen using a cholesteric, reflective polarizer or the like as a reflective polarization element in combination with a diffusing element. This projection screen does not reflect part of the environmental light incident on it because of the polarized-light-separating property of the cholesteric, reflective polarizer or the like, and diffuses the reflected light by means of the diffusing element that is provided separately from the cholesteric, reflective polarizer.
Namely, in the projection screens described in Patent Document 2, only a specific polarized component of the imaging light projected from a projector is selectively reflected by making use of the so-called polarized-light-separating property in order to sharply produce an image, and, moreover, the imaging light reflected is diffused in order to improve image visibility.
The polarized-light-separating layers for use in the projection screens described in Patent Documents 1 and 2, however, are made of organic films containing cholesteric liquid crystals and so forth and therefore contain molecules of liquid crystals that are organic compounds. Such liquid crystalline molecules have a marked tendency to absorb light in a wave range extending on the shorter wavelength side, so that they excessively absorb light in the blue (B) color wave range, which extends, of the wave ranges of light of the three primary colors, on the shortest wavelength side, if imaging light (containing light in the wave ranges for red (R), green (G) and blue (B) colors, the three primary colors of light) is emitted from a conventional projector. For this reason, the projection screens described in Patent Documents 1 and 2 show decreased efficiency of reflection of light in the blue (B) color wave range contained in the imaging light projected; these projection screens provide lowered image visibility because they cannot sharply display blue color.